I. Field of the Invention
The present invention relates to a mechanical part made of ceramics as a rocker arm, ball valve, tappet, piston, etc. for an automotive engine.
In the case of an automotive engine part, the positional and form tolerances are severely limited. For this reason, it is necessary to grind the contact surface of the part in order to remove the distorsion caused by baking or firing. In this instance, a grinding wheel of a relatively coarse grain size of #200 or so (according to Japanese Industrial Standards) is used in order to increase the cutting rate.
However, when the contact surface of the part is ground coarse or rough, the actual contact area of the part becomes small. This causes an increased surface pressure and wear of a mating surface of another mechanical part which is made of metal equal to or lower in mechanical property than ceramics. In order to prevent this, a grinding wheel of a finer grain size is used to finish grind the contact surface and thereby reduce the roughness height and spacing after shaping and sizing of the mechanical part by a grinding wheel of a coaser grain size.
A problem of the prior art grinding process is that it is not suited for mass production since each mechanical part must be not only rough ground but finish ground independently, i.e., both of rough grinding and finish grinding must be made to the parts one by one.
Another problem is that the mechanical part which is finish ground according to the prior art grinding process has a difficulty of holding lubricant on its contact surface due to the small roughness height and spacing.